As one convenient measure for public nuisance of the waste gas exhausted from a heat engine such as an automobile internal engine, a tertiary catalyst system has been suitably employed to clear up at once the desired regulations of nitrogen oxides (NO.sub.x), carbon monooxide (CO) and hydrocarbons. In the tertiary catalyst system, the characteristics of the tertiary catalyst may not have effective function unless an air/fuel ratio (A/F ratio) is controlled in a very narrow range of equivalent ratio or a theoretical air/fuel ratio. The A/F ratio is conveniently controlled in such a way that the oxygen content after combustion is measured by the oxygen sensor as a change in the oxygen partial pressure and the measured signal is fed back to the control system of the electronic fuel injection pump for example. To measure the change in the oxygen partial pressure, various types of oxygen sensors have been developed employing the transition-metal oxide in which the electric resistance is varied depending on the oxygen concentration.
The oxygen sensor of this type may be prepared by mixing for example titanium dioxide material powder with suitable solvent, binder, plasticizer, dispersant and the like forming a green sheet by the doctor blade technique for subsequent lamination with a platinum electrode and a heater under the pressure with dryness and then sintering the resultant product to form a porous disc. This process is disclosed in the opened Japanese Patent application No. 50-56292. In order to actually mount this element for an exhaust tube of the automobile, a platinum lead wire is lead out through a passage provided in a ceramic body fixed to a metal housing with a possible seal preventing the gas leakage from the sensor element to the atmosphere. The actual layout of such the element is technically disclosed in the opened Japanese Utility Model Applications No. 52-46781 and No. 52-4792.
However, the structure of the known element is relatively inferior in strength with such the disadvantage that a boundary part of the lead wire prolonged from the element becomes weak against oscillation and impact. Further, the prolonged platinum wire necessitates a passage to be formed as a groove or hole in the ceramic with relatively the high cost in production and difficulty in sealing for the gas leakage.